1. Field of the Invention
The present invention relates to a control apparatus of a synchronous motor, and in particular to a control apparatus of a synchronous motor for preventing demagnetization in the synchronous motor during a dynamic braking operation.
2. Description of the Related Art
Dynamic braking is used to perform emergency stop of a synchronous motor. A brief description will now be made of dynamic braking. In FIG. 1A, there is shown a state of connection between a synchronous motor (hereinafter, referred to simply as “motor”) and a driving amplifier during driving of the motor. A motor 100 is connected to a driving amplifier 200 via power lines 300, so that a current flows through a three-phase coil (not illustrated) of the motor 100. FIG. 1B is a voltage vector diagram illustrated on a d-q coordinate during operation of the motor, wherein Id is a d-phase current, Iq is a q-phase current, Vd is a d-phase voltage, Vq is a q-phase voltage, Ld is a d-axis inductance, Lq is a q-axis inductance, ω is a frequency, Kv is a counter electromotive voltage constant, and R is a winding resistance.
When dynamic braking operation is performed, as illustrated in FIG. 2A, the motor 100 is disconnected from the driving amplifier 200 so that the power lines 300 connected to the three-phase coil of the motor 100 are placed in a shorted state, and a current that is caused to flow through the three-phase coil due to an induced voltage of the motor 100 is used as a control current to brake the motor 100. FIG. 2B is a voltage vector diagram illustrated on a d-q coordinate during dynamic braking operation of the motor. During the dynamic braking operation, the voltage across terminals of the motor 100 becomes zero (0) [V]. Consequently, the q-phase current Iq (torque generation current), which is in opposite direction so as to cancel the counter electromotive voltage Kvω, flows in the motor so that braking occurs.
Upon initiation of the dynamic braking operation, the voltage across terminals is abruptly varied from a large value to 0 [V]. Thus, immediately after the dynamic braking (hereinafter, also referred to as “DB”) is initiated, the current amplitude become oscillatory. Changes over time in the d-phase current Id and q-phase current Iq after the initiation of DB are illustrated in FIGS. 3A and 3B. FIG. 3A illustrates a case in which the initial q-phase current Iq immediately after the initiation of DB at the time 0 [sec] is 0 [A]. FIG. 3B illustrates a case in which the initial q-phase current Iq is large. Let it be assumed, for the sake of convenience, that the q-phase current Iq (torque generation current) accelerates the motor when being positive on the graph and decelerates the motor when being negative. Further, let it be assumed that the d-phase current Id (field weakening current) acts to weaken the field when being positive on the graph.
As illustrated in FIG. 3A, even when the initial q-phase current Iq is 0 [A], the d-phase current Id oscillates, but the current amplitude is relatively small. In contrast, when the initial q-phase current Iq is large as illustrated in FIG. 3B, the amplitude of the d-phase current Id becomes larger than when the initial q-phase current Iq is 0 [A]. Especially, at a time tm immediately after initiation of DB, the d-phase current Id is increased momentarily, so that there is a possibility that demagnetization is caused in the permanent magnet used for the rotor of the motor.
There are methods for suppressing demagnetization from being caused due to the dynamic braking operation (for example as described in, Japanese Patent No. 5616409, Japanese Laid-open Patent Publication No. 2013-099210, and Japanese Patent No. 5113395 referred to hereinafter as “Patent Documents 1-3”, respectively. In Patent Document 1, there is disclosed a method for controlling a current of a synchronous motor such that the current value of the synchronous motor becomes less than a maximum current value in order to prevent irreversible demagnetization of a permanent magnet which could be caused by a transient current occurring in the case of three-phase short-circuit. However, there is a problem in that the torque is limited unnecessarily since the q-phase current Iq is limited beforehand to suppress the current.
In Patent Document 2, there is disclosed a motor control apparatus including three switching elements configured, when turned on, to cause three phase coils of a three-phase motor to be short-circuited and when turned off, to eliminate the short-circuit of the three phase coils, wherein in the case where a current flowing through any one phase coil of the three phase coils exceeds a predetermined threshold value when the three switching elements are in a turned-on state, the three switching elements are turned off for a first predetermined period of time, and turned on after a lapse of the first predetermined period of time. However, since it occurs after the current of the coils exceeds the predetermined threshold value that the short-circuiting of the coils is eliminated, it is difficult to completely suppress demagnetization due to dynamic braking operation.
In Patent Document 3, there is disclosed a dynamic braking device for a motor including the dynamic braking device for causing the motor to be emergency-stopped, wherein during a predetermined period of time after a dynamic braking operation is initiated, a control is performed such that a dynamic braking current is weakened via application braking with a PWM control in which at least part of switching elements to control the brake current are repetitively turned on and off, and after lapse of the predetermined period of time, a control is performed such that a dynamic braking current, which should originally flow, is permitted to flow with all the switching elements being fixed in the on or off state at all times without executing the PWM control. However, since, in this case as well, it is after the dynamic braking operation is initiated that the dynamic braking current is controlled to be weakened, it is difficult to completely suppress demagnetization due to dynamic braking.